1. Field of the Invention
This invention relates to improved two phase anaerobic digestion having separated acid and methane forming phases which converts biomass to desired methane product gas with high efficiency and enhances the methane content of product gas by feeding the gas product and the liquid product of the acid forming phase to separated methane phase digesters.
2. Description of the Prior Art
Anaerobic digestion has been known to stabilize sludge and other predominantly organic materials, and usable product gas, of varying composition, has been obtained from such anaerobic digestion processes. The organic feed mixture which provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. The process of anaerobic digestion biodegrades any of these organic carbonaceous materials, under appropriate operating conditions, to product gas which contains desirable methane gas.
Separated two phase anaerobic digestion processes have been found to enhance the conversion efficiency, such as described in Pohland and Ghosh, Biotechnol and Bio-Eng. Symp. No. 2, 85-106 (1971), John Wiley & Sons, Inc., and by the same authors in Environmental Letters, 1 (4), 255-266 (1971), Marcel Dekker, Inc. In an acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous matter to volatile fatty acids of low molecular weight. The volatile fatty acids remain solubilized in the liquid portion of the digester contents. The liquid and solid effluent from the acid phase is conveyed to a methanae second phase, where methanogenic microorganisms convert the volatile fatty acids to product gas composed primarily of methane and carbon dioxide. Product gas is removed from the methane phase and processed, or scrubbed, to separate the methane component which is drawn off as pipeline gas. The gas separation of methane is an expensive process which detracts from the economic feasibility of the anaerobic biodegradation of organic carbonaceous material to produce methane gas.
U.S. Pat. No. 4,022,665 discloses certain specific operating conditions for a two phase anaerobic digestion process, such as feed rates and detention times, which promote efficient conversion of organic material. Additionally, the '665 patent discloses two separated methane phases, a methane phase II operated in series with methane phase I. The methane phase II receives effluent fluid and/or effluent gas from the methane phase I. The improved process of the present invention employs two discrete methane producing reactors which operate in parallel. All improvements disclosed in U.S. Pat. No. 4,022,665 can be adapted for use according to the improved process of this invention and the teachings of that patent are incorporated herein by reference.
U.S. Pat. No. 3,383,309 teaches that the rate and efficiency of the anaerobic digestion process, particularly in the methane forming phase, are increased when hydrogen gas is introduced into the digester sludge. According to the '309 patent, hydrogen gas is introduced into both the acid forming and the methane forming phases, to increase the availability of energy-rich "hypersludge" . A portion of the product gas removed from the reactor is thermally cracked in a gas reformer and molecular hydrogen and carbon monoxide are returned to the digester.